Combustion systems in industrial facilities consume huge amounts of energy and in the process create waste byproducts. Industrial combustion systems include boilers, heaters, kilns, furnaces, gas or steam turbines, and reciprocating engines. These systems use thermal energy from burning fuel to transfer heat for materials processing or to produce mechanical energy. Much of the world's convertible energy comes from fossil fuels that are burned to produce heat that is then used as a transfer medium into mechanical energy or other means to generate electricity or accomplish other tasks.
Fossil fuels are any naturally occurring organic fuel, such as petroleum, coal, and natural gas. Fuels used in industrial combustion equipment include natural gas, heavy oil, light oil, fossil fuel by-products, waste fuels and wood waste. Coal is the most abundant fossil fuel in the U.S. Coal found in the eastern U.S. typically has high-sulfur and is found in deep deposits. Conversely, western coal is of low-sulfur content found closer to the surface.
Commercial petroleum fuels are divided into grades which are based on the fuel viscosity. Fuel oil is widely used in power plants. Crude oil can also be used directly, but this is not as desirable. Some of the advantages of oil over coal are that it is cleaner, easier to handle, store and transport, and produces less ash. Also, oil can be atomized and mixed with combustion air to ease burning. One disadvantage of oil is that the ash is very adhesive and difficult to remove. Some oils are high in sulfur which is also difficult to remove. Oil can contain vanadium, which once oxidized, causes corrosion of ferrous materials found in most boilers and furnaces.
Natural gas consists mostly of methane and ethane. Liquefied petroleum gas (LPG) is primarily composed of propane and butane. Natural gas is the easiest of the fossil fuels to burn as it mixes well with air and burns cleanly with little ash. Natural gas has the highest heating value of all fossil fuels, but has the lowest known fuel reserves and requires cryogenic tanks for storage.
Fossil fuels come from the long-term decomposition of plant and animal matter from millions of years ago. Therefore, these fuels are slow to regenerate and are considered a non-renewable resource. Combustion of fossil fuels creates exothermic reactions between carbon, hydrogen and sulfur with oxygen. The byproducts of this combustion process include carbon dioxide (CO2) and sulfurous oxides (SOX). Disassociation reactions with nitrogen (N2), oxygen (O2), and carbon dioxide can occur if the temperature of the process is high enough. These reactions create nitrous oxides (NOX) and carbon monoxide (CO). These resulting gas emissions increase the “greenhouse” effect leading to global warming. These emissions also cause many direct air pollution problems like poor air quality, smog, and, indirectly, acid rain.
Fossil fuels, particularly natural gas, will provide a major portion of world energy requirements over the next several decades. Environmental and cost considerations drive the development of higher efficiency and cleaner burning energy conversion systems. Whereas an internal combustion engine converts about 40% of its fuel into usable power, a typical coal burning power plant extracts only up to 35% of coal's potential energy. More recent development of a hybrid combined cycle plant provides an overall efficiency gain in power generation. Here, the combustion gases from coal or natural gas are first used to drive a gas turbine powering a generator. The exiting gases are then sent to a heat recovery steam unit also generating energy, and lastly to the exhaust stack. The thermal efficiency of this system can reach 60%, but has been limited to intermediate power generation applications.
A combustion system can also be used as an incinerator with the main purpose to dispose or transform the input fuel or added materials. These systems use intense heat to breakdown targeted material for disposal. The input materials are reduced through state changes, oxidation, or other heat induced chemical reactions. The resulting component ingredients are further processed, filtered, and separated into material forms that can be handled for recycling or final disposal. Disposal may include depositing solids into a known waste site such as a land fill or discharging waste water or gases into the environment.
It is clear, industrial combustion systems present some of the most cost-effective opportunities for obtaining significant reductions in contaminant emissions and energy efficiency improvements. Toward this end, some attempts have been made to recycle combustion exhaust gases. In more contained industrial applications, past efforts to recirculate combustion gases have directed only a small percentage of the gas back from the exhaust stream with most of the gas being sent to the exhaust stack. The amount of heat lost can be significant in a large industrial boiler or incinerator because the combustion gases must also be heated to operational temperature in the combustion chamber thereby absorbing heat energy and reducing efficiency. In most cases, the oxygen content of the gas used for combustion is around 20%. Therefore, there is energy lost in heating the remaining 80% even though it does not contribute to the process. In some cases, oxygen enrichment has improved efficiency and pre and post combustion heat exchangers are used to minimize heat loss. However, use of exhaust gas recirculation has been limited because of problems associated with contamination in the exhaust stream with the need to maintain a proper combustion concentration of oxygen. Typically, gas recirculation adds combustion system complexity without significant overall advantage.